I think you're better off thinking about how to mitigate the conditions. The CPU does a nice job of heating itself so if you case the teensy in a small box with 1/2" or thicker XPS) or similar removable rigid foam), the teensy should be ok.
If not, simply add a 200mW heater to the VUSB / GND pins and place it over the crystal. Ideally that heater would be self regulating. I've seen them used in refrigerator mullion strips, for example.
FWIW, I have successfully used a teensy 3 to -44*C and beyond with dry ice. That said, the teensy started at room temperature or was already on when it was operating at those temperatures.
The error of the RTC can be compensated as long as you have a good idea re the ambient temperatures and have characterized the crystal ppm error vs temperature response. I recommend using a ds18b20 or similar digital temperature sensor.
For inspiration, search for my temperature experiments here to see what I have posted about. On iPhone right now so finding and inserting the links is a bit tedious.